On a railcar, wheeled trucks are provided toward and support opposite ends of a railcar body for movement over tracks. Each truck includes a bolster extending essentially transversely of the car body longitudinal centerline for supporting the railcar body. In the preponderance of freight cars, a pivotal connection is established between the bolster and railcar body by center bearing plates and bowls transversely centered on the car body underframe and the truck bolster. Accordingly, the truck is permitted to pivot on the center bearing plate under the car body. As the railcar moves between locations, the car body tends to adversely roll from side to side.
Attempts have been made to control the adverse roll of the railcar body through use of side bearings positioned on the truck bolster outwardly of the center bearing plate. A “gap style” side bearing has been known to be used on slower moving tank/hopper railcars. Conventional “gap style” side bearings include a metal, i.e. steel, block or pad accommodated within a pocket defined on the truck bolster. An upstanding housing or cage, integrally formed with or secured, as by welding or the like, to the truck bolster defines the pocket and inhibits sliding movement of the metal block relative to the bolster. The pockets provided on the bolster can, and often do, differ in size relative to each other. As is known, a gap or vertical space is usually present between the upper surface of the “gap style” side bearing and the underside of the railcar body.
Under certain dynamic conditions, combined with lateral track irregularities, the railcar truck also tends to oscillate or “hunt” in a yaw-like manner beneath the car body. The coned wheels of each truck travel a sinuous path along a tangent or straight track as they seek a centered position under the steering influence of the wheel conicity. As a result of such cyclic yawing, “hunting” can occur as the yawing becomes unstable due to lateral resonance developed between the car body and the truck. As will be appreciated, excessive “hunting” can result in premature wear of the wheeled truck components including the wheels, bolsters, and related equipment. Hunting can also furthermore cause damage to the lading being transported in the car body.
Track speeds of rail stock, including tank/hopper cars, continues to increase. Increased rail speeds translate into corresponding increases in the amount of yaw or hunting movements of the wheeled trucks. As will be appreciated, “gap style” side bearings cannot and do not limit hunting movements of the wheeled trucks. As such, the truck components including the wheels, bolsters, and related equipment tend to experience premature wear.
Constant contact side bearings for railcars are also known in the art and typically include a base and cap. The base has a cup-like configuration and is suitably fastened to the bolster. The cap is biased from the base and includes an upper surface for contacting and rubbing against an underside of the car body. As will be appreciated, the cap is free to vertically move relative to the side bearing base. Such constant contact side bearings furthermore includes a spring.
The spring for such side bearings can comprise either spring loaded steel elements or elastomeric blocks or a combination of both operably positioned between the side bearing base and the cap. The purpose of such spring is to resiliently urge the upper surface of the cap under a preload force and into frictional contact with the car body underframe. Elastomeric blocks appear to advantageously offer a more controlled friction at the interface of the side bearing cap and the car body underframe, preclude seizing, and create a less rigid shear constraint whereby permitting the wheeled trucks to negotiate minor track irregularities without breaking friction at the interface between the side bearing cap and the car body underframe. One such elastomeric block is marketed and sold by the Assignee of the present invention under the tradename “TecsPak.”
Known constant contact side bearings are simply not designed to fit or be accommodated within existing pockets on a truck bolster of a railcar. The base of a typical constant contact side bearing includes attachment flanges or lugs radially extending from opposed sides of the base for securing the bearing assembly to the railcar truck bolster. Accordingly, to use a constant contact side bearing on railcar having a bolster with a pocket requires either replacement of the entire truck bolster or complete removal of the upstanding housing or cage, defining the pocket, from the surface of the bolster to which the attachment flanges or lugs of the side bearing are secured. Either proposal requires extensive manual efforts and, thus, is expensive while keeping the railcar out of service for an extended time period.
Some railcar designs further exacerbate the problem of fitting a constant contact side bearing thereto. In many railcar designs, a constant contact side bearing operates within a five and one-sixteenth inch nominal working space between the truck bolster and the car body underside. Such dimension usually provides sufficient space for the spring to develop the required preload force for the side bearing. In other railcar designs (i.e., tank/hopper railcars), however, the vertical space between the bolster, to which the side bearing is secured, and the car body underside is severely restricted. In fact, some railcar designs provide only about a two and five-eights inch nominal working space between the truck bolster and the underside of the railcar. The reduced work space envelope provided on many railcar designs is too limited to accommodate a constant contact side bearing to control such hunting movements.
Additionally, heat buildup in proximity to an elastomeric spring of constant contact side bearings is a serious concern. While advantageously producing an opposite torque acting to inhibit the yaw motion of the truck, the resulting friction between the side bearing and underside of the car body develops an excessive amount of heat. The repetitive cyclic compression of the elastomeric block coupled with high ambient temperatures, in which some railcars operate, further exacerbate spring deformation. As will be appreciated, such heat buildup often causes the elastomeric block to soften/deform, thus, significantly reducing the ability of the side bearing to apply a proper preload force whereby decreasing vertical suspension characteristics of the side bearing resulting in increased hunting.
Thus, there is a continuing need and desire for a constant contact railcar side bearing design capable of use with railcar truck bolsters having a pocket for accommodating the side bearing and which is capable of effective operation in limited space constraints without serious deterioration on a long term basis.